1. Field of the Invention
This invention relates to an improved process for converting coal or similar solid carbonaceous material containing certain "scale" forming components or precursors. More particularly, this invention relates to an improved process for liquefying coal and similar carbonaceous materials.
2. Description of the Prior Art
As is well known, coal has long been used as a fuel in many areas. For several reasons, such as handling problems, waste disposal problems, pollution problems and the like, coal has not been a particularly desirable fuel from the ultimate consumers point of view. As a result, oil and gas have enjoyed a dominant position, from the standpoint of fuel sources, throughout the world.
As is also well known, proven petroleum and gas reserves are shrinking throughout the world and the need for alternate sources of energy is becoming more and more apparent. One such alternate source is, of course, coal since coal is an abundant fossil fuel in many countries throughout the world. Before coal will be widely accepted as a fuel, however, it is believed necessary to convert the same to a form which will not suffer from the several disadvantages alluded to previously.
To this end, several processes wherein coal is either liquefied and/or gasified have been proposed heretofore. Of these, the processes wherein coal is liquefied appear to be more desirable in most cases since a broader range of products is produced and these products are more readily transported and stored. Difficulty has, however, been encountered during the liquefaction of certain coals, particularly the lower ranking coals, apparently as the result of extraneous metals or metal components contained in these coals.
While the inventors here do not wish to be bound by any particular theory, it is believed that the operating difficulties are associated with the presence of one or more alkaline earth metals, particularly calcium, and to some extent the presence of iron, which react during liquefaction with available anions to form a solid scale or deposit. As liquefaction continues, the amount of scale increases in the liquefaction reactor thereby reducing reactor volume, and hence, the liquefaction contacting time and/or the total throughput. Ultimately, complete plugging may occur. Moreover, it is possible that portions of the scale or deposits can dislodge from the walls and result in downstream plugging.
The scaling and/or deposit problem is believed to have been first reported upon in the literature in connection with the operation of a high pressure coal liquefaction plant for producing liquids from lignites at Wesseling, near Cologne, Germany. According to the literature, operation of this plant was severely limited by a solid referred to as "caviar", the reference apparently stemming from the appearance of the solid in the form of agglomerated balls or spherulites. According to the literature, the spherulites were found to comprise calcium carbonate and hexagonal crystals of iron sulfide.
Early attempts to solve the problem involved the use of what might be termed engineering techniques which were designed either to prevent scale formation or to remove the scale before operating problems were encountered. In one such technique, a small slipstream was withdrawn from an initial reactor of a series in a process. With this technique, the initially formed particles were continuously withdrawn and removed and the slipstream then returned to the reactor. This technique aided in suppressing further crystal growth and slowed down the rate of scale formation within the reactor. The technique did, however, result in high gas losses and erosion rates within auxiliary equipment.
More recently, it has been discovered that calcium carbonate deposits which form during liquefaction as the result of the decomposition of various calcium organic compounds can be avoided by converting the calcium organic compounds which do decompose during liquefaction to a salt which will remain stable during liquefaction or to a form which can be removed prior to liquefaction. Conversions of this type can be effected with a relatively broad range of pretreating agents including salts of metals different from calcium which will, effectively, replace the calcium in the coal, various organic and inorganic acids and certain gaseous pretreating agents such as SO.sub.2 and SO.sub.3.
For the most part, these ion exchange-type pretreatments have been quite effective in solving the scale or deposition problem. Most such treatments involve the use of pretreating agents which are known as pollutants and therefore which must be separated from any gas stream ultimately vented to the atmosphere and some of which are hazardous in their own right. The need, therefore, for an improved method of avoiding the scale and/or solid deposition problem is believed to be readily apparent.